Optical Coherence Tomography Control Systems and Methods

ABSTRACT

In part, the invention relates to methods, devices, and systems suitable for controlling a light source. The light source is configured for use in a data collection system such as an optical coherence tomography system. The light source can be controlled with a drive waveform. Linearizing and symmetrizing parameters of the light source such as forward and backward scan durations is achieved using a suitable drive waveform. Phase, amplitude, and other parameters for different harmonics of a fundamental wave can be identified that improve operating parameters such as the duty cycle and peak frequency matching between scans. The fundamental wave and one or more of such harmonics can be combined to generate the suitable drive wave form. The light source can include a tunable light source that includes or is in optical communication with a tunable filter.

RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationNo. 61/695,399 filed on Aug. 31, 2012, the disclosure of which is hereinincorporated by reference in its entirety.

BACKGROUND

Optical coherence tomography (OCT) is an interferometric imagingtechnique with widespread applications in ophthalmology, cardiology,gastenterology and other fields of medicine. Huang D, Swanson E A, Lin CP, Schuman J S, Stinson W G, Chang W, Hee M R, Flotte T, Gregory K,Puliafito C A, and Fujimoto J G, “Optical coherence tomography,”Science, Vol 254, 1178-1181 (1991). The ability to view subsurfacestructures with high resolution (2-15 μm) through small-diameterfiber-optic probes makes OCT especially useful for minimally invasiveimaging of internal tissues and organs.

Time-domain OCT systems employ a broadband light source as an input toan interferometer with a mechanically actuated reference arm forpath-length scanning. The interference signals generated by reflectionsfrom structures at different depths are measured point-by-point as thereference path length changes. In this measurement scheme, the maximumscanning speed is limited both by the dynamic mechanical constraints ofthe actuator and by the spectral power density of the light source. Insuch a system using a superluminescent light source that emits an outputpower of 25 mW over a spectral bandwidth of 40-60 nm, the maximumdepth-scanning velocity that can be achieved while maintaining anadequate signal-to-noise ratio for tissue imaging (>90 dB) isapproximately 25 m/s. Therefore, 512-line images of a 5 mm deep objectcan be acquired at a rate no greater than 10 per second.

Frequency-domain (also called Fourier-domain) (FD) OCT overcomes thesespeed constraints by taking advantage of optical frequencydiscrimination methods based on Fourier transformation, which eliminatethe need for long-range mechanical actuators. Swanson E A and Chinn S R,“Method and Apparatus for Performing Optical Frequency DomainReflectometry” U.S. Pat. No. 6,160,826 (issued Dec. 12, 2000), Choma MA, Sarunic M V, Yang C, and Izatt J, “Sensitivity advantage of sweptsource and Fourier domain optical coherence tomography,” Opt. Express,Vol. 11, 2183-2189 (2003).

Instead of wasting available source power by interrogating the samplepoint-by-point, FD-OCT collects information from multiple depthssimultaneously and discriminates reflections from different depthsaccording to the optical frequencies of the signals they generate.FD-OCT systems based on swept-frequency light sources have attracted themost attention for medical applications that require subsurface imagingin highly scattering tissues.

The feasibility of swept-source OCT (SS-OCT) has been demonstrated inseveral academic research studies. Chinn S R, Swanson E A, and FujimotoJ G, “Optical coherence tomography using a frequency-tunable opticalsource,” Opt. Lett. Vol. 22, 340-342 (1997); Yun S H, Tearney G J, BoumaB E, Park B H, de Boer J F, “High-speed spectral domain opticalcoherence tomography at 1.3 μm wavelength,” Optics Express, Vol. 11, pp.3598-3604 (2003); Choma M A, Hsu K, and Izatt J, “Swept source opticalcoherence tomography using an all-fiber 1300 nm ring laser source,” J.Biomed. Optics, Vol. 10, p. 044009 (2005); Huber R. Wojtkowski, Taira K,Fujimoto J G, and Hsu K. “Amplified, frequency-swept lasers forfrequency domain reflectometry and OCT imaging: design and scalingprinciples,” Opt. Express, Vol. 13, 3513-3528 (2005). Various SS-OCTsystems have been reported including systems based on Fourier DomainMode Locked (FDML) lasers, surface emitting cavity lasers, short cavitylasers, long cavity lasers, and tunable lasers that include a tunablefilter.

Some of the implementations disclosed to date suffer from drawbacks thathave discourage widespread commercialization of SS-OCT. For example,certain implementations make real-time data acquisition and displaydifficult, because they employ data acquisition schemes that requirepost-acquisition re-sampling or interpolation of recorded data beforeFourier transformation. In addition, the relatively short coherencelength and tendency for mode-hopping of short-cavity lasers reducesignal-to-noise and image resolution at optical scan depths exceeding2-3 mm. Many medical applications, including coronary artery imaging,require an optical scan depth that exceeds 5 mm. Some OCT system andmethod implementations designed to address some of the problems outlinedabove relating to SS-OCT in context of tunable light sources are recitedin Schmitt “Method and Apparatus for Swept-Source Optical CoherenceTomography” U.S. Pat. No. 7,916,387 (issued Mar. 29, 2011) and Schmitt“Method and Apparatus for Swept-Source Optical Coherence Tomography”U.S. Pat. No. 8,325,419 (issued Dec. 12, 2012).

Other approaches relating to SS-OCT and the light sources used with suchsystems have been proposed. For example, SS-OCT systems using avertical-cavity surface-emitting laser (VCSEL) laser are described inJayaraman “System for Swept Source Optical Coherence Tomography” U.S.Pat. No. 7,468,997 (issued Dec. 23, 2008) and Chong “Swept Source TypeOptical Coherent Tomography System” U.S. Pat. No. 7,701,588 (issued Apr.20, 2010).

In general, for SS-OCT systems, it is often the case that the sweptlight sources that are used exhibit nonlinear sweep patterns. As aresult, the optical frequency of light generated by the source does notchange linearly over each sweep period. These light sources may alsosuffer from asymmetry in the forward and backward scans. Further, thenonlinearity and asymmetry problems tend to vary from light source tolight source, which makes it difficult, if not impossible, to apply thesame corrective action to every unit. A need therefore exists to addresssuch problems. In part, the embodiments described herein address theseproblems and others relating to certain light sources.

SUMMARY

In part, one embodiment of the invention relates to light sources thatare responsive to a time varying electrical signal alternativelyreferred to herein as a drive waveform or drive signal. The lightsources can be configured for use with a data collection system and oneor more associated data collection probes. The drive waveform can beused to control a swept light source such as by controlling a tunablefilter. In one embodiment, the data collection system is an opticalcoherence tomography (“OCT”) system. In part, the invention relates tomethods suitable for use with light sources that are in opticalcommunication with or otherwise include a tunable filter that has anasymmetric and/or nonlinear response to a drive waveform.

In one embodiment, a plurality of waveforms, such as sinusoids, arecombined to create a waveform suitable for driving a light source or acomponent thereof such as a tunable filter. In one embodiment, thecombination waveform is configured to reduce or correct an asymmetricsweep response and a nonlinear sweep response simultaneously. That is,instead of improving the asymmetric sweep response of the light sourcewhile the nonlinear sweep response stays the same or worsens or does notimprove to a satisfactory level (or vice versa with respect to improvingthe nonlinear sweep response while the asymmetric sweep response staysthe same or worsens or does not improve to a satisfactory level), bothsymmetry and linearity of the sweep response of the light source areimproved by the drive waveform.

In one embodiment, a filter or light source has or produces a wavelengthtuning profile in one sweep direction that is not a mirror image of theprofile in the other sweep direction even though a symmetric drivewaveform is applied to the filter. This is an example of a type ofasymmetric sweep response. For example, application of a sinusoidaldrive waveform, which is symmetric in time about a point halfway throughits repetition period, may result in a forward sweep that is longer induration than the backward sweep. In one embodiment, a filter or lightsource lacks or otherwise does not produce a tuning profile that islinear in optical frequency with time. This is an example of a type ofnonlinear sweep response. For example, application of a sinusoidal drivewaveform may result in forward and backward sweep profiles that aresinusoidal in optical wavelength with time.

In one embodiment, the light source is a vertical-cavitysurface-emitting laser (VCSEL), a FDML light source, or a tunable lightsource that includes or is in optical communication with a tunablefilter. The filter can include a MEMs device in one embodiment. Thefilter can include a piezoelectric component in one embodiment. In oneembodiment, the second harmonic is tuned to correct asymmetry and thethird harmonic is tuned to correct nonlinearity in the forward andbackward scans. In one embodiment, the second harmonic is tuned tocorrect nonlinearity and the third harmonic is tuned to correctasymmetry in the forward and backward scans. The tuning can be performedby modifying the second harmonic and the third harmonic by adjusting oneor more of their respective phases or amplitudes.

In one aspect, the invention relates to a method of controlling a lightsource having a tunable filter. The method includes generating a firstharmonic wave having a first frequency, wherein the first harmonic wavehas a first amplitude indicative of a voltage and a first phase;generating a second harmonic wave having a second frequency, wherein thesecond harmonic wave has a second amplitude indicative of a voltage anda second phase; generating a third harmonic wave having a thirdfrequency, wherein the third harmonic wave has a third amplitudeindicative of a voltage and a third phase; generating a modified secondharmonic wave; generating a modified third harmonic wave; andsuperpositioning the first harmonic wave, the modified second harmonicwave and the modified third harmonic wave to generate a drive waveformconfigured to substantially symmetrize and substantially linearize asweep response of the light source.

In one embodiment, the method further includes storing the drivewaveform in memory. In one embodiment, the step of generating a modifiedsecond harmonic wave includes substituting the second phase with afourth phase such that the modified second harmonic results; and thestep of generating a modified third harmonic wave includes substitutingthe third phase with a fifth phase such that the modified third harmonicwave results. In one embodiment, the step of generating a modified thirdharmonic wave includes substituting the third amplitude with a fourthamplitude such that the modified third harmonic wave results. In oneembodiment, the method further includes driving the tunable filter withthe drive waveform and storing optical coherence data. In oneembodiment, the light source is a vertical-cavity surface-emitting laser(VCSEL). In one embodiment, the method further includes increasing aneffective duty cycle of the VCSEL to be greater than or equal to 90% ofa sweep period of the light source by driving the tunable filter withthe drive waveform. In one embodiment, the step of generating a modifiedsecond harmonic wave includes substituting the second amplitude with afifth amplitude such that the modified second harmonic wave results.

In one embodiment, the method further includes adjusting a constantvoltage bias of the drive waveform such that a center wavelength of thelight source is about 1310 nm. In one embodiment, the method furtherincludes adjusting an alternating current gain of the drive waveformsuch that a tuning range of the light source is about 100 nm. In oneembodiment, configuring the waveform to substantially symmetrize andsubstantially linearize a sweep response includes the steps ofgenerating a sweep response that has (i) a scan duration in the forwardscan direction that differs from a scan duration in the backward scandirection by less than about 15% and (ii) a peak RF frequency in theforward scan direction that differs from a peak RF frequency in thebackward scan direction by less than about 15%.

In one aspect, the invention relates to an optical coherence tomographysystem. The system includes a drive waveform source that includes amemory, an output, and one or more function generators configured to (a)generate a first harmonic wave, (b) generate a second harmonic wave (c)generate a third harmonic wave; and (d) combine the first, second, andthird harmonic waves such that a drive waveform is generated and storedin the memory; and a light source that includes a tunable filter, thetunable filter in electrical communication with the output andconfigured to receive the drive waveform from the output, wherein thesecond harmonic has a first phase value and/or a first amplitude valueconfigured to cause a peak radiofrequency (RF) OCT signal frequency fora backward scan of the tunable filter and a peak RF OCT signal frequencyfor a forward scan of the tunable filter to differ by less than about15%.

In one embodiment, the drive waveform source is a control system, aprocessor or a circuit. In one embodiment, the light source is a VCSEL.In one embodiment, the light source is an FDML laser or any tunablelight source incorporating a filter with an asymmetric or nonlinearresponse. In one embodiment, wherein the third harmonic has a secondphase value and a second amplitude value configured to minimize a peakRF OCT signal frequency for a backward scan and a forward scan. Acontrol system, a processor or a circuit can be used to generate one ormore functions such as waveforms and combine them to generate the drivewaveforms specified herein.

In one embodiment, the first phase value is generated by searching foran extremum in a two dimensional space that includes forward andbackward scan data measured with respect to the light source as thetunable filter is swept. In one embodiment, the first phase amplitude isgenerated by searching for an extremum in a two dimensional space thatincludes forward and backward scan data measured with respect to thelight source as the tunable filter is swept. In one embodiment, theeffective duty cycle of the VCSEL is greater than about 90% of a sweepperiod of the VCSEL when collecting OCT data.

In one embodiment, the VCSEL has a tuning range of about 100 nm. In oneembodiment, a forward scan duration of the tunable filter is about equalto a backward scan duration of the tunable filter. In one embodiment, aratio of a forward scan duration to a backward scan duration ranges fromabout 0.8 to about 1.2 when the drive waveform is applied to the tunablefilter. In one embodiment, a ratio of a peak RF OCT signal frequencyduring a forward scan to a peak RF fringe frequency during a backwardscan duration ranges from about 0.9 to about 1.1 when the drive waveformis applied to the tunable filter.

In one aspect, the invention relates to a method of controlling anoptical coherence tomography system. The optical coherence tomographysystem includes a light source comprising a tunable filter. The methodincludes generating a first harmonic wave having a first frequency,wherein the first harmonic wave has a first amplitude indicative of avoltage and a first phase: generating a second harmonic wave having asecond frequency, wherein the second harmonic wave has a secondamplitude indicative of a voltage and a second phase; generating a thirdharmonic wave having a third frequency, wherein the third harmonic wavehas a third amplitude indicative of a voltage and a third phase;generating a modified second harmonic wave; generating a modified thirdharmonic wave; superpositioning the first harmonic wave, the modifiedsecond harmonic wave and the modified third harmonic wave to generate adrive waveform; and generating a sweep response for the light sourcecomprising a scan duration in a forward scan direction that differs froma scan duration in a backward scan direction by less than about 15% bydriving the tunable filter with the drive waveform.

In one aspect, the invention relates to an optical coherence tomographysystem. The system includes a swept light source that includes a tunablefilter that includes an input, the tunable filter in opticalcommunication with a sample arm of an interferometer, wherein thetunable filter is drivable bidirectionally; a control system thatincludes a non-transitory memory and an output, wherein the output is inelectrical communication with the input, the control system configuredto drive the tunable filter in one or more directions; and a drivewaveform stored in the non-transitory memory, the drive waveformconfigured to substantially symmetrize and substantially linearize asweep response of the swept light source, wherein an effective dutycycle of the swept light source is greater than about 90% of a sweepperiod of the swept light source when collecting optical coherencetomography data.

BRIEF DESCRIPTION OF DRAWINGS

The figures are not necessarily to scale, emphasis instead generallybeing placed upon illustrative principles. The figures are to beconsidered illustrative in all aspects and are not intended to limit theinvention, the scope of which is defined only by the claims.

FIG. 1A shows an optical coherence tomography system with a light sourcedriven by a drive waveform according to an illustrative embodiment ofthe invention.

FIG. 1B shows a method of designing and/or generating a drive waveformsuitable for use with a light source in an optical coherence tomographysystem in accordance with an illustrative embodiment of the invention.

FIG. 2 shows an oscilloscope screenshot depicting various parameters ofa drive waveform that includes a fundamental wave in accordance with anillustrative embodiment of the invention.

FIG. 3A shows a plot of drive amplitude versus time with respect to adrive waveform according to an illustrative embodiment of the invention.

FIG. 3B shows a plot of measured signal frequency versus time withrespect to a drive waveform according to an illustrative embodiment ofthe invention.

FIG. 3C shows a plot of expected signal frequency versus time withrespect to a drive waveform according to an illustrative embodiment ofthe invention.

FIG. 4A shows a plot of the peak RF fringe frequency versus phase of adrive waveform that includes a second harmonic according to anillustrative embodiment of the invention.

FIG. 4B shows a plot of the sweep scan duration versus phase of a drivewaveform that includes a second harmonic according to an illustrativeembodiment of the invention.

FIG. 5 shows an oscilloscope screenshot depicting various parameters ofa drive waveform that includes a second harmonic wave in accordance withan illustrative embodiment of the invention.

FIG. 6A shows a plot of drive amplitude versus time with respect to adrive waveform composed of two harmonics according to an illustrativeembodiment of the invention.

FIG. 6B shows a plot of measured signal frequency versus time withrespect to a drive waveform according to an illustrative embodiment ofthe invention.

FIG. 6C shows a plot of the peak RF fringe frequency versus amplitude ofa drive waveform that includes a third harmonic according to anillustrative embodiment of the invention.

FIG. 6D shows a plot of the peak RF fringe frequency versus phase of adrive waveform that includes a third harmonic according to anillustrative embodiment of the invention.

FIG. 7 shows an oscilloscope screenshot depicting various parameters ofa drive waveform that includes a third harmonic wave in accordance withan illustrative embodiment of the invention.

FIG. 8A shows a plot of drive amplitude versus time with respect to adrive waveform composed of three harmonics according to an illustrativeembodiment of the invention.

FIG. 8B shows a plot of measured signal frequency versus time withrespect to a drive waveform according to an illustrative embodiment ofthe invention.

FIGS. 9A-9C show three respective drive waveforms of increasingsuitability for use with an optical coherence tomography system inaccordance with an illustrative embodiment of the invention.

DETAILED DESCRIPTION

In part, the invention relates to methods, systems and devices fordetermining a suitable drive waveform for use with an electromagneticradiation source alternatively referred to herein as a light source suchthat the drive waveform improves one or more operational parameters ofthe light source. In one embodiment, the light source is a swept sourcesuch as a swept source laser. The light source is typically configuredfor use with a data collection system such as an OCT system. In oneembodiment, the light source is a VCSEL, a Fourier Domain Mode Lockedlaser, or a light source incorporating a tunable filter with anasymmetric response, a nonlinear response or both an asymmetric andnon-linear response. In one embodiment, the tunable filter is drivablebidirectionally.

Intravascular OCT imaging systems, such as the system 5 of FIG. 1A,include a light source 7 such as a wavelength-swept light source. In oneembodiment, the light source includes or is in optical communicationwith a tunable filter 9. The tunable filter 9 can be a device such as aMEMs device that it is in optical communication with the light source 7.The tunable filter 9 can be a component of the light source 7. Sweptlight sources, such as source 7, generally produce alternating series of“forward” (short to long wavelength) and “backward” (long to shortwavelength) scans as a tunable filter 9 of the light source 7 is drivenwith a drive waveform 10. In one embodiment, the drive waveform 10 is anoscillating, periodic drive waveform such as a sinusoidal voltagewaveform.

As discussed in more detail below, the drive waveform 10 can beconfigured to increase the effective duty cycle of the light source 7.In one embodiment, effective duty cycle refers to the percentage of timeduring the period of the drive waveform where OCT image data can becollected. The effective duty cycle can vary based on the quality of thelaser output, the RF OCT signal frequency, and the sampling rate of theOCT digitizer system. In addition, the drive waveform 10 can include aplurality of component waves used to generate a resultant waveform suchas by Fourier synthesis. In one embodiment, the drive waveform 10includes three component waves that are selected such that when combinedsymmetry and linearity of the output light source 7 are improvedrelative to other types of input waveforms. The drive waveform 10 can beused to control light source 7 or a component in optical communicationwith the light source 7.

Certain swept light sources that are suitable for use in OCT systemssuffer from inherently nonlinear sweep patterns, whereby the opticalfrequency of light generated by the source, such as source 7, does notincrease or decrease linearly over each sweep period. Furthermore, theselight sources may also suffer from inherent asymmetry in the forward andbackward scans, whereby the scan produced in one direction may becompressed or distorted compared to the other direction due to theproperties of the tunable filter used in the light source. Since thetunable filter, such as filter 9, used in each individual light sourcehas unique response characteristics, the nature of the nonlinearity andasymmetry problems can vary from unit to unit, making it impossible toapply the same corrective action to every unit. As a result, drivewaveforms 10 can be designed on a per light source basis to improve theoperation of a given light source in a given OCT system. A drivewaveform source 15 can include a processor, one or more functiongenerators, a non-transitory memory, a suitable control system or othersuitable electronic devices configured for generating and configuringdrive waveforms for transmission to the tunable filter 9.

In one embodiment, the drive waveform source 15 is a control system thatincludes a non-transitory memory and an output such as a port orcontact. The tunable filter can be in optical communication with asample arm of an interferometer in one embodiment. The tunable filtercan include an input such as port or a contact configured to receive acontrol signal such as a drive waveform 10. The tunable filter isdrivable bidirectionally. Although, in some embodiments, the drivewaveform 10 can be used to drive the tunable filter in one direction.The drive waveform 10 can also be used to drive the tunable filter inone or more directions. In one embodiment, the output is in electricalcommunication with the input. The drive waveform can be generated for aparticular light source and stored in the non-transitory memory orcreated by the waveform source and stored in a non-transitory memory. Inone embodiment, the drive waveform is configured to substantiallysymmetrize and substantially linearize a sweep response of the sweptlight source. In one embodiment, the effective duty cycle of the sweptlight source is greater than about 90% of a sweep period of the sweptlight source when collecting optical coherence tomography data.

A drive waveform 10 that includes a plurality of related or harmonicsine waves configured to correct and/or improve upon both thenonlinearity and asymmetry found in certain swept light sources 7 andmethods of determining such as drive waveform are both embodiments ofthe invention. An OCT system with such a drive waveform or drivewaveform source 15 is also one embodiment of the invention. In otherembodiments, a stepwise method for determining the relative weightingand phase offsets of the constituent sine waves is used such that anoptimal waveform can be selected for an individual light source.

As shown in FIG. 1A, the OCT system 5 includes a light source 7, such asa wavelength swept light source, which can include a tunable filter 9. Adrive waveform is determined based on one or more parameters of thelight source 7. Typically, a plurality of parameters are evaluated, suchas phase, amplitude, sweep duration, peak RF OCT signal frequency, andother scan direction specific parameters, as part of the process ofdetermining a suitable drive waveform 10. In one embodiment, twoparameters are evaluated simultaneously to improve linearity andsymmetry of the sweep response. The drive waveform 10 is transmitted tothe light source 7 through an electrical connection 12 such as one ormore wires, a bus, or other data transmission mechanism, in oneembodiment. The waveform source 15 is in electrical communication withlight source 7, such as for example, by being in electricalcommunication with a tunable filter 9. If the swept source 7 does notinclude a tunable filter, the drive waveform source 15 can be inelectrical communication with another component of the light source 7having an input configured to receiving a drive waveform 10.

In one embodiment, the light source 7 is in optical communication withan interferometer 18. As shown, an optical path 17 such as one or moreoptical fibers can be used to facilitate optical communication betweenthe light source 7 and the interferometer 17. The interferometer may bea Michelson interferometer. A probe 23 such as an OCT data collectionprobe can be in optical communication with interferometer 18 alonganother optical path 21 such as another optical fiber as shown. In oneembodiment, the probe includes a rotatable fiber 25 disposed within acatheter. The rotatable fiber 25 can be in optical communication withoptical path 21. The probe 23 can be sized for insertion into a sample27 or lumen of interest, such as a blood vessel or artery. In oneembodiment, the probe 25 receives electromagnetic radiation directedfrom the light source 7 to the interferometer 18. The probe in turndirects light into a sample 27, such as a blood vessel, and collectsreflected light from the blood vessel. The interferometer 18 is used inaccordance with OCT principles to generate depth information based onthe light from the source 7 and the light collected from the sample 27.

A drive waveform source 15 such as a control system can be used togenerate a suitable drive waveform to control the tunable filter 9and/or the light source 7 in one embodiment. The output response of alight source 7 for different input drive waveforms 22, 55, 70,respectively, are shown in FIGS. 2, 5, and 7. The waveform can besynthesized by combining the outputs of three sine wave generators.Alternatively, an arbitrary waveform generator can be used to directlysynthesize the waveform. In either case, in one embodiment, a generalformula for the waveform f(t)=V_(DC)+A₀ sin(2πf₀t+Φ₀)+A₁sin(2πf₁t+Φ₁)+A₂-sin(2πf₂t+Φ₂), where V_(DC) is a DC bias voltage,f₂=3f₀ and f₁=2f₀. For example, A₀ can range from between about 0 V toabout 200 V, f₀ can range from about 25 kHz to about 2 MHz.

The various function generators described herein such as sine wavegenerators can be implemented using circuits, devices, and softwaremodules. The various function generators can be part of the drivewaveform source 15 or in electrical or optical or wireless communicationwith it. The drive waveform source 15 can include a processor such as acentral processing unit or a microprocessor or an ASIC with instructionsto generate a suitable drive waveform Alternatively, the drive waveformsource can include or be able to access memory that includes apreviously generated and stored waveform suitable for a given lightsource or tunable filter. The invention is not limited to the use ofsine wave generators or sine waves, but can be implemented using variousdrive wave forms generated by combining harmonic functions, non-harmonicfunctions, and other functions in various embodiments to provide asuitable drive waveform as outlined herein.

In one embodiment, the light source 7 is a tunable laser that includesor is in optical communication with a tunable filter. In part, oneembodiment of the invention relates to a method to derive a drivewaveform suitable for use with a light source 7 such that a suitablelevel of symmetry and/or linearity or an increased duty cycle resultssuch as an optimized level or maximum level. Three harmonic waves, suchas sinusoids or sine waves, are combined to generate the drive waveformin one embodiment. In one embodiment, the fundamental frequency of thefirst wave (the first harmonic) is about 100 KHz. The frequencies of thesecond wave (second harmonic) and third wave (third harmonic) can beinteger multiples of the fundamental frequency of the first wave. In oneembodiment, the frequency of the second wave is about 200 kHz. In oneembodiment, the frequency of the third wave is about 300 kHz.

A suitable drive waveform configured for use with a light source such asa laser can be designed or generated on a per light source basis. Onemethod 30 for generating such a drive waveform is shown in FIG. 1B.Initially, the first step is generating the fundamental sine wave Step1. In addition, another step, which may be optional in some embodiments,is setting the constant voltage bias Step 1 a to obtain a desired centerwavelength of the wavelength sweep. Another step, which may be optionalin some embodiments, is setting the amplitude of fundamental sine waveStep 1 b to obtain a desired tuning range. These steps may be performediteratively until a target DC bias and/or target amplitude is reachedthat yield an optimal or otherwise suitable drive waveform. The nextstep is generating the second harmonic Step 2.

In one embodiment, the amplitude of the second harmonic is adjusted aswell as its phase. As a result, additional steps can include setting theamplitude of the second harmonic Step 2 a and setting the phase of thesecond harmonic Step 2 b to substantially symmetrize the forward scanand the backward scan with respect to time and frequency or to obtainmaximum time and frequency symmetry between forward and backward scans.The forward and backward scans of the light source are discussed in moredetail below. The next step is generating the third harmonic Step 3.Additional steps can include setting the amplitude of the third harmonicStep 3 a and setting the phase of the third harmonic Step 3 b tosubstantially linearize the scan or obtain a suitable level of scanlinearity. The final step shown is to output a drive waveform Step 4 asshown.

The steps shown in FIG. 1B and otherwise described herein can beperformed in different orders with all, some or none of Steps 1 a, 1 b,2 a, 2 b, 3 a, and 3 b. For example, in one embodiment the secondharmonic can be configured to substantially linearize the light sourcescan while the third harmonic can be configured to substantiallysymmetrize the light source scan. Additionally, some or all of the stepsmay be repeated iteratively in order to optimize the light source scan.Adjustments to the amplitude and phase of the various harmonics can beused to configure the waveform to improve certain aspects of the sweepresponse such as its symmetry and linearity.

FIG. 2 is an oscilloscope screenshot showing various waves and relatedfrequency data associated with a VCSEL light source. FIG. 5 and FIG. 7show additional oscilloscope screenshots with different drive waveforms.The drive waveforms in FIGS. 5 and 7 include waveform 22 with the secondharmonic added (FIG. 5) and the third harmonic added to waveform 22 andthe second harmonic (FIG. 7), respectively, or modified versionsthereof. The drive waveform 55 of FIG. 5 and the drive waveform 70 ofFIG. 7 are suitable for driving a VCSEL light source or a componentthereof.

The VCSEL light source includes a tunable filter. The top half of thescreenshot of FIG. 2 shows the RF OCT signals, commonly referred to asinterference fringes 33 and 35, generated by a Mach-Zehnderinterferometer (MZI) when a forward and backward wavelength scan from aVCSEL is used as the interferometer input. The path length differencebetween the two arms of the MZI was set to about 16.0 mm in air. Withthis arrangement, the MZI generates interference signals thatapproximate the signals which would be generated during OTF imaging witha Michelson interferometer at an imaging range of about 8.0 mm in air,which is a desirable range for imaging of blood vessels. In this case,the drive waveform 22 comprises a DC bias voltage and a pure sinusoidwith a frequency of 100 kHz and is configured to generate a tuning rangeof about 100 nm centered near a wavelength of about 1310 nm. Theinterference fringe amplitude 33 during the backward scan is attenuatedcompared to the forward scan in part because the RF signal frequencyexceeds the bandwidth of the detector electronics.

From the screenshot, it is clear that the VCSEL response is highlyasymmetric at 100 kHz drive frequency. The drive waveform 22 correspondsto a sine wave as the fundamental waveform or first harmonic. Thisfundamental waveform is a time varying voltage as shown and is used todrive a tunable filter used in the VCSEL. In the top half of FIG. 2, theMZI output generated by the VCSEL in response to the input drivewaveform 22 is shown. The amplitude along the vertical axis is in unitsof about 200 millivolts per unit division and time is shown along thehorizontal axis in units of 1 microsecond per unit division.

Optical devices such as tunable filters can be driven with a raisedcosine waveform. See Spectrum of Externally Modulated Optical Signals,Ho and Kahn. Journal of Lightwave Technology, Vol. 22, No. 2, February2004. Such a drive waveform can include a rapid “flyback” section and anelongated raised cosine section. Under this tunable filter drivecondition, the flyback section is intentionally not useable for OCTimaging. Instead, a raised cosine drive waveform attempts to minimizethe duration of the flyback while maximizing the duration and linearityof the forward scan. Unfortunately the raised cosine method does notachieve these goals for certain VCSEL designs when the drive waveformfrequency is 100 kHz, which is a desirable frequency for operation OCTsystem. The methods, systems, and device described herein relating todrive waveform design and generation using multiple harmonic sinusoidsaddresses these limitations of the raised cosine waveform.

As discussed above with respect to FIG. 1A, swept light sources, such asthe VCSEL produce alternating series of “forward” (short to longwavelength) and “backward” (long to short wavelength) scans as thetunable filter is driven with a drive waveform. In FIG. 2, theinterference fringes from a “backward” scan 33 is shown on the left inthe top half of the figure while the “forward” scan 35 is shown on theright side. As discussed herein, the backward scan 33 shown alsocorresponds to the flyback portion of the VCSEL response to an inputdrive waveform. The tunable filter used in the VCSEL has a highlynonlinear and asymmetric voltage response when driven at frequenciesaround 100 kHz. A frequency spectrum is shown in the bottom half of thefigure and below waveform 22. The units for the frequency data in thebottom portion of FIGS. 2, 5 and 7 use decibels for the vertical axisand frequency for the horizontal axis.

As shown in the oscilloscope screenshot of FIG. 2 (and in FIGS. 5 and 7)the bottom portion of each screenshot generally shows a radiofrequencyspectra generated using a rectangular Fast Fourier Transform (FFT)window sized to 90% of the duration of a single sweep direction (i.e., aforward direction or a backward direction). In FIG. 2, the windowed FFTof forward scan 35 is shown as indicated by brackets 45. In addition, afrequency marker shown as a dotted line 48 corresponds to the peakradiofrequency of the backward scan. This marker 48 shows the highestfrequency of the compressed backward scan, 462.9 MHz, which includes anexcessive amount of radiofrequencies due to the highly time-compressednature of the backward scan.

Another marker 50 associated with the forward scan identifies a peakfrequency value of 246.88 MHz. It is desirable to have the frequencycontent of the forward scan and the backward scan to be the same. It isalso desirable to minimize the peak frequency contained in theinterference fringes generated during OCT imaging. If the frequencycontent of the two scans is significantly different, data acquisition ismore complicated and a more complex data acquisition (DAQ) device isrequired. Additionally, a higher peak signal frequency necessitates afaster data acquisition device to accurately sample the data, whichincreases the system cost and complexity.

In FIG. 2, the frequency content differs significantly, as is clear frommarkers 48, 50, while in FIG. 7 it is more closely matched between theforward and backward scan as a result of the application of a suitablyconfigured drive waveform. In addition, the peak signal frequencygenerated during the entire sweep period that includes a forward andbackward scan is substantially reduced. Using the drive waveform of FIG.2 would likely necessitate using a more expensive and complex DAQrelative to the drive waveform of FIG. 7. For example, it may benecessary to use a 1 GS/s digitizer in order to sample the OCT signalsgenerated at an imaging depth of about 8.0 mm. By using harmonics of adrive waveform, a better operating range for acquiring data is possible.The oscilloscope screens shots of FIGS. 2, 5 and 7 are obtained with aMach-Zehnder interferometer (MZI) mismatch equivalent to about 8.0 mmimaging range.

FIG. 3A shows a plot of a 100 kHz sinusoidal drive waveform. The forward(FWD) scan and the backward (BWD) scan portions are identified. Thevertical axis is for the drive amplitude and the horizontal axis is fortime. The normalized alternating current (AC) component of the drivewaveform is shown in the plot of FIG. 3A. The AC gain and DC bias wereadjusted to obtain a tuning range of about 100 nm centered near about1310 rm.

FIG. 3B shows the measured instantaneous interference fringe frequencyusing the drive waveform of FIGS. 3A and 2 applied to a VCSEL. The VCSELis part of an OCT system that generates interference fringes as part ofthe interferometric measurements. The fringe frequency is directlyproportional to the location of a sample in a sample arm of aninterferometer relative to the location of a reference reflector in areference arm of an interferometer. An MZI with a fixed path mismatch ofabout 16.0 mm in air was used to generate the interference fringes,corresponding to an OCT imaging range of about 8.0 mm in air. In oneembodiment, a light source comprising a tunable filter is in opticalcommunication with an interferometer having a sample arm and areference. The light source is configured as described herein withregard to one or more of a duty cycle, a sweep period, a response orother parameters described herein such that the light source is suitablefor performing swept source OCT data collection. The light source or acomponent thereof can be connected to a control system or clockingsystem in one embodiment.

As shown in FIG. 3B, the forward and backwards scans are highlyasymmetric. The vertical axis measures fringe frequency and thehorizontal axis measures time. The duration of the forward scan of about6.25 microseconds is longer than backward scan duration of about 3.75microseconds. As a result, the ratio of the duration of the forward scanto the backward scan is about 6.25/3.75 or about 1.67. In turn, the peakradiofrequency of the forward scan of about 244 MHz is less than thepeak radiofrequency of the backward scan of about 461 MHz. As a result,the ratio of the peak radiofrequency of the forward scan to the peakradiofrequency of the backward scan is about 244 MHz/461 MHz or about0.529.

FIG. 38 also shows a DAQ Cutoff line corresponding to the maximum fringefrequency of 275 MHz that can be sampled by a DAQ system incorporating a550 megasample/second digitizer. The backward scan exceeds the DAQCutoff, making it unusable for OCT imaging with this digitizer. Theforward scan is below the DAQ Cutoff, making it usable for OCT imagingwith this digitizer. Since only one scan direction is usable, theeffective line rate of the OCT system would be 100 kHz and the effectiveduty cycle would be about 62.5%. FIG. 3B also shows an ideal linecorresponding to the fringe frequency that would be obtained with aperfectly linear and symmetric wavelength sweep. The backward scandeviates substantially from this best-case situation.

FIG. 3C shows the expected instantaneous frequency given the 100 kHzapplied drive waveform of FIG. 2 and FIG. 3A. The nonlinear VCSELresponse yields a significantly different frequency profile compared toexpectation from the applied drive waveform. The plots of FIGS. 3B and3C demonstrate the need to find a suitable drive waveform to overcomethese issues. As described with respect to FIG. 1B, using harmonics ofthe 100 kHz fundamental wave to tailor a drive waveform allows for manyof these negative features to be improved upon or corrected.

Given the limitations associated with using a sine wave as a drivewaveform without further modifications, it is useful to add a secondharmonic to the fundamental harmonic. In addition, it is also useful tomodify the second harmonic by changing its amplitude and phase. FIG. 4Ashows the phase optimization of the second harmonic with respect to peakRF frequency. FIG. 48 shows the phase optimization of the secondharmonic with respect to sweep scan duration. Phase is measured inradians as shown. Each of FIGS. 4A and 4B define a respective twodimensional (2D) space or a combination 2D space.

The second harmonic (200 kHz) wave, such as a sine wave, is used tosubstantially symmetrize forward and backward scans. One or more 2Dspaces of amplitude and phase are searched to a find point of maximumsymmetry, such as a relative maximum or extremum or an absolute maximumor extremum for one or more parameters. In one embodiment, analysis andsearching of data sets, such as 2D data sets corresponding to FIGS. 4Aand 4B, using maximum or extermum identifying techniques identifiedabout −1.83 radians as suitable phase selection for the second harmonic.A phase value of about −1.83 radians is suitable for use with the secondharmonic because the peak RF frequency and the sweep scan duration areboth close to equal with this phase value. Thus, a second harmonic canundergo a phase adjustment to generate a modified second harmonic. Inone embodiment, the process of identifying an optimal second harmonicamplitude and phase is automated by searching the 2D space for the pointof maximum symmetry. Automation can be achieved throughcomputer-controlled waveform generation and frequency measurements. Thesearch may be exhaustive within defined limits of amplitude and phase,or may incorporate conventional search optimization algorithmswell-known in the art.

FIG. 5 is an oscilloscope screenshot showing various waves and relatedfrequency data associated with a VCSEL light source. The drive waveform55 shown in FIG. 5 includes the waveform 22 of FIG. 2 with a secondharmonic thereof added to it after the second harmonic is optimized asdiscussed above with respect to FIGS. 4A and 4B. The forward scanduration 60 and the backward scan duration 63 are substantiallyidentical. The frequency marker at peak RF of backward scan 65 has afrequency of about 331.63 MHz. Similarly, the frequency marker at peakRF of forward scan 67 has a frequency of about 324.49 MHz. From theshape of the uppermost scans and these measurements, there are nearlyidentical peak RF frequencies and scan durations which result afteroptimization of the second harmonic's amplitude and phase. As a result,the ratio of the peak radiofrequency of the forward scan to the peakradiofrequency of the backward scan is about 324.49 MHz/331.63 MHz orabout 0.978.

FIG. 6A shows a plot of a sinusoidal drive waveform that includes thesecond harmonic drive waveform of FIG. 5 and the fundamental harmonicdrive waveform of FIG. 2. The forward (FWD) scan and the backward scanportions are identified. The vertical axis is for the drive amplitudeand the horizontal axis is for time. The normalized alternating current(AC) component of the drive waveform is shown in the plot of FIG. 6A.The AC gain and DC bias were readjusted to obtain a tuning range ofabout 100 nm centered near about 1310 nm.

FIG. 6B shows the measured instantaneous frequency using the drivewaveform of FIGS. 6A and S applied to a VCSEL. As shown, the forward andbackwards scans are highly symmetric. The vertical axis measures fringefrequency and the horizontal axis measures time. The duration of theforward scan of about 5.16 microseconds is slightly longer the backwardscan duration of about 4.84 microseconds. The ratio of the forward scanduration to the backward scan duration is about 1.157. In turn, the peakradiofrequency of the forward scan of about 320 MHz is only slightlyless than the peak radiofrequency of the backward scan of about 327 MHz.The ratio of these two frequency (forward/backward) is about 0.979. Inaddition, the forward scan and the backward scan are both approachingthe DAQ cutoff which would result in a useable line rate of 200 kHz andan effective duty cycle of about 100%. The next step to modify the drivewaveform of FIG. 5 is to add a third harmonic to further increase itssuitability for use with a given light source for an OCT system.

FIG. 6C shows the amplitude optimization of the third harmonic withrespect to peak RF frequency. FIG. 6D shows the phase optimization ofthe third harmonic with respect to peak RF frequency. Phase is measuredin radians as shown. Each of FIGS. 6C and 6D defines a respective twodimensional (2D) space or a combination 2D space. The third harmonic(300 kHz) wave, such as a sine wave, is used to substantially linearizethe forward and backward scans. One or more 2D spaces of amplitude andphase are searched to a find point of minimum RF frequency, such as arelative minimum or extremum or an absolute minimum or extremum for oneor more parameters.

In one embodiment, analysis and searching of data sets, such as 2D datasets corresponding to FIGS. 6C and 6D, using maximum or extermumidentifying techniques identified an amplitude of about 0.12 and a phaseof about −1.01 radians. This amplitude and phase were identified becausethe peak RF frequency is substantially minimized with these values ofamplitude and phase. These values substantially linearize the responseof a given light source such as a tunable laser, for example, when thedrive waveform includes the fundamental wave, the second harmonic, andthe third harmonic with the parameters described herein or variations insuch parameters as is possible with one or more of the optimizationsteps.

FIG. 7 is an oscilloscope screenshot showing various waves and relatedfrequency data associated with a VCSEL light source. The drive waveform70 shown in FIG. 7 includes the waveform 55 of FIG. 5 added to the thirdharmonic after it is optimized as discussed above with respect to FIGS.6C and 6D. The peak RF frequencies in both scan directions are minimizedand equalized after application of third harmonic (300 kHz). As shown,the forward scan duration 75 and the backward scan duration 77 aresubstantially identical. The frequency marker at peak RF of backwardscan 80 has a frequency of about 285.31 MHz. Similarly, the frequencymarker at peak RF of forward scan 83 has a frequency of about 280.27MHz. The ratio of 280.27 MHZ to 285.31 MHz is about 0.98. The closenessof these values and the minimization of these values for the drivewaveform of FIG. 7 shows a significant improvement relative to thefrequency marker at peak RF of the backward scan having a frequency ofabout 462.9 MHz in FIG. 2 while the frequency marker at peak RF of theforward scan had a frequency of about 246.88 MHz to yield a comparativeratio of about 1.875 (backward/forward) or about 0.53(forward/backward).

FIG. 8A shows a drive waveform generated using three sine waves usingone or more of the steps of FIG. 1B. The AC component has beennormalized. In addition, the AC gain and DC bias were readjusted toobtain a 100 nm tuning range centered near 1310 nm.

FIG. 8B shows the measured instantaneous frequency using the drivewaveform of FIGS. 7 and 8A applied to a VCSEL. The forward and backwardscans are symmetric and have minimum peak RF frequency. The duration ofthe forward scan of about 5.00 microseconds is the same as the backwardscan duration of about 5.00 microseconds, thus the comparative ratiois 1. In turn, the peak radiofrequency of the forward scan of about 268MHz is about the same as the peak radiofrequency of the backward scan ofabout 275 MHz. In addition, the both the forward scan and the backwardscan are at or below the DAQ cutoff resulting in a useable line rate of200 kHz and an effective duty cycle of about 100%.

FIGS. 9A-9C show three plots of the drive waveforms of FIGS. 2, 5, and7, respectively. The amplitude along the vertical axis is in units ofabout 300 millivolts per unit division and time is shown along thehorizontal axis in units of 1 microsecond per unit division The changesin the drive waveform over time occur as a result of the addition of thedifferent optimized harmonics as discussed herein.

In one embodiment, the DC bias of the fundamental wave ranges from about0 to about 200 V. In one embodiment, the amplitude of the fundamentalwave ranges from about 0 to about 200 V. In one embodiment, the tuningrange ranges from about 20 to about 200 nm. In one embodiment, the phaseof the fundamental wave ranges from about 0 to about 2π radians. In oneembodiment, the phase of the second harmonic ranges from about 0 toabout 2π radians. In one embodiment, the phase of the third harmonicranges from about 0 to about 2π radians. In one embodiment, theamplitude of the second harmonic ranges from about 0 to about 200 V. Inone embodiment, the amplitude of the third harmonic ranges from about 0to about 200 V.

In one embodiment, the drive waveform described herein is configured tohave a 100% duty cycle at 100 kHz drive frequency. In one embodiment,the drive waveform described herein is configured to have substantiallysymmetric or symmetric forward and backward sweeps. In one embodiment,the drive waveform described herein is configured to have a 200 kHzeffective sweep rate. In one embodiment, the drive waveform describedherein is configured to have a RF signal frequency ≦ about 275 MHz atabout 8 mm imaging range and about a 100 nm tuning range.

In the description, the invention is discussed in the context of opticalcoherence tomography; however, these embodiments are not intended to belimiting and those skilled in the art will appreciate that the inventioncan also be used for other imaging and diagnostic modalities or opticalsystems in general.

The terms light and electromagnetic radiation are used interchangeablyherein such that each term includes all wavelength (and frequency)ranges and individual wavelengths (and frequencies) in theelectromagnetic spectrum. Similarly, the terms device and apparatus arealso used interchangeably. In part, embodiments of the invention relateto or include, without limitation: sources of electromagnetic radiationand components thereof; systems, subsystems, and apparatuses thatinclude such sources; mechanical, optical, electrical and other suitabledevices that can be used as part of or in communication with theforegoing; and methods relating to each of the forgoing. Accordingly, asource of electromagnetic radiation can include any apparatus, matter,system, or combination of devices that emits, re-emits, transmits,radiates or otherwise generates light of one or more wavelengths orfrequencies.

One example of a source of electromagnetic radiation is a laser. A laseris a device or system that produces or amplifies light, by the processof stimulated emission of radiation. Although the types and variationsin laser design are too extensive to recite and continue to evolve, somenon-limiting examples of lasers suitable for use in embodiments of theinvention can include tunable lasers (sometimes referred to as sweptsource lasers), superluminescent diodes, laser diodes, semiconductorlasers, mode-locked lasers, gas lasers, fiber lasers, solid-statelasers, waveguide lasers, laser amplifiers (sometimes referred to asoptical amplifiers), laser oscillators, and amplified spontaneousemission lasers (sometimes referred to as mirrorless lasers orsuperradiant lasers).

The aspects, embodiments, features, and examples of the invention are tobe considered illustrative in all respects and are not intended to limitthe invention, the scope of which is defined only by the claims. Otherembodiments, modifications, and usages will be apparent to those skilledin the art without departing from the spirit and scope of the claimedinvention.

The use of headings and sections in the application is not meant tolimit the invention; each section can apply to any aspect, embodiment,or feature of the invention.

Throughout the application, where compositions are described as having,including, or comprising specific components, or where processes aredescribed as having, including or comprising specific process steps, itis contemplated that compositions of the present teachings also consistessentially of, or consist of, the recited components, and that theprocesses of the present teachings also consist essentially of, orconsist of, the recited process steps.

In the application, where an element or component is said to be includedin and/or selected from a list of recited elements or components, itshould be understood that the element or component can be any one of therecited elements or components and can be selected from a groupconsisting of two or more of the recited elements or components.Further, it should be understood that elements and/or features of acomposition, an apparatus, or a method described herein can be combinedin a variety of ways without departing from the spirit and scope of thepresent teachings, whether explicit or implicit herein.

The use of the terms “include,” “includes,” “including,” “have,” “has,”or “having” should be generally understood as open-ended andnon-limiting unless specifically stated otherwise.

The use of the singular herein includes the plural (and vice versa)unless specifically stated otherwise. Moreover, the singular forms “a,”“an,” and “the” include plural forms unless the context clearly dictatesotherwise. In addition, where the use of the term “about” is before aquantitative value, the present teachings also include the specificquantitative value itself, unless specifically stated otherwise.

It should be understood that the order of steps or order for performingcertain actions is immaterial so long as the present teachings remainoperable. Moreover, two or more steps or actions may be conductedsimultaneously.

Where a range or list of values is provided, each intervening valuebetween the upper and lower limits of that range or list of values isindividually contemplated and is encompassed within the invention as ifeach value were specifically enumerated herein. In addition, smallerranges between and including the upper and lower limits of a given rangeare contemplated and encompassed within the invention. The listing ofexemplary values or ranges is not a disclaimer of other values or rangesbetween and including the upper and lower limits of a given range.

Non-Limiting Software Embodiments for Implementing Drive WavefrontGeneration

The present invention may be embodied in many different forms,including, but in no way limited to, computer program logic for use witha processor (e.g., a microprocessor, microcontroller, digital signalprocessor, or general purpose computer), programmable logic for use witha programmable logic device, (e.g. a Field Programmable Gate Array(FPGA) or other PLD), discrete components, integrated circuitry (e.g.,an Application Specific Integrated Circuit (ASIC)), or any other meansincluding any combination thereof. In one embodiment of the presentinvention, some or all of the processing of the data used to generate ordesign a drive waveform or component thereof is implemented as a set ofcomputer program instructions that is convened into a computerexecutable form, stored as such in a computer readable medium, andexecuted by a microprocessor under the control of an operating system.Control and operation of components of a given light source, such as alaser, such as a VCSEL, can also be so controlled or operated using acomputer. In one embodiment, light source, drive waveform, controlsystem data, or tunable filter parameters are transformed into processorunderstandable instructions suitable for generating drive signals fortunable filters, operating a light source for an OCT system with asuitable duty cycle, controlling a tunable filter, signal processing,function generation, sweeping a tunable filter in a first direction orin a first direction and a second direction and other features andembodiments as described above.

Computer program logic implementing all or part of the functionalitypreviously described herein may be embodied in various forms, including,but in no way limited to, a source code form, a computer executableform, and various intermediate forms (e.g., forms generated by anassembler, compiler, linker, or locator). Source code may include aseries of computer program instructions implemented in any of variousprogramming languages (e.g., an object code, an assembly language, or ahigh-level language such as Fortran, C. C++. JAVA, or HTML) for use withvarious operating systems or operating environments. The source code maydefine and use various data structures and communication messages. Thesource code may be in a computer executable form (e.g., via aninterpreter), or the source code may be converted (e.g., via atranslator, assembler, or compiler) into a computer executable form.

The computer program may be fixed in any form (e.g., source code form,computer executable form, or an intermediate form) either permanently ortransitorily in a tangible storage medium, such as a semiconductormemory device (e.g., a RAM, ROM, PROM, EEPROM, or Flash-ProgrammableRAM), a magnetic memory device (e.g., a diskette or fixed disk), anoptical memory device (e.g., a CD-ROM), a PC card (e.g., PCMCIA card),or other memory device. The computer program may be fixed in any form ina signal that is transmittable to a computer using any of variouscommunication technologies, including, but in no way limited to, analogtechnologies, digital technologies, optical technologies, wirelesstechnologies, networking technologies, and internetworking technologies.The computer program may be distributed in any form as a removablestorage medium with accompanying printed or electronic documentation(e.g., shrink-wrapped software), preloaded with a computer system (e.g.on system ROM or fixed disk), or distributed over a network.

Programmable logic may be fixed either permanently or transitorily in atangible storage medium, such as a semiconductor memory device (e.g., aRAM, ROM. PROM, EEPROM, or Flash-Programmable RAM), a magnetic memorydevice (e.g., a diskette or fixed disk), an optical memory device (e.g.,a CD-ROM), or other memory device. The programmable logic may be fixedin a signal that is transmittable to a computer using any of variouscommunication technologies, including, but in no way limited to, analogtechnologies, digital technologies, optical technologies, wirelesstechnologies (e.g., Bluetooth), networking technologies, andinternetworking technologies. The programmable logic may be distributedas a removable storage medium with accompanying printed or electronicdocumentation (e.g., shrink-wrapped software), preloaded with a computersystem (e.g., on system ROM or fixed disk), or distributed from a serveror electronic bulletin board over the communication system (e.g., theInternet or World Wide Web).

Various examples of suitable processing modules are discussed below inmore detail. As used herein a module refers to software, hardware, orfirmware suitable for performing a specific data processing or datatransmission task. Typically, in a preferred embodiment a module refersto a software routine, program, or other memory resident applicationsuitable for receiving, transforming, routing and processinginstructions, or various types of data such as OCT scan data, functiongenerator data, harmonic wave data, tunable filter data, frequencies,interferometer signal data, filter drive signals, linear drive signals,and other information of interest.

Computers and computer systems described herein may include operativelyassociated computer-readable media such as memory for storing softwareapplications used in obtaining, processing, storing and/or communicatingdata. It can be appreciated that such memory can be internal, external,remote or local with respect to its operatively associated computer orcomputer system.

Memory may also include any means for storing software or otherinstructions including, for example and without limitation, a hard disk,an optical disk, floppy disk, DVD (digital versatile disc), CD (compactdisc), memory stick, flash memory, ROM (read only memory), RAM (randomaccess memory), DRAM (dynamic random access memory), PROM (programmableROM), EEPROM (extended erasable PROM), and/or other likecomputer-readable media.

In general, computer-readable memory media applied in association withembodiments of the invention described herein may include any memorymedium capable of storing instructions executed by a programmableapparatus. Where applicable, method steps described herein may beembodied or executed as instructions stored on a computer-readablememory medium or memory media.

It is to be understood that the figures and descriptions of theinvention have been simplified to illustrate elements that are relevantfor a clear understanding of the invention, while eliminating, forpurposes of clarity, other elements. Those of ordinary skill in the artwill recognize, however, that these and other elements may be desirable.However, because such elements are well known in the art, and becausethey do not facilitate a better understanding of the invention, adiscussion of such elements is not provided herein. It should beappreciated that the figures are presented for illustrative purposes andnot as construction drawings. Omitted details and modifications oralternative embodiment, are within the purview of persons of ordinaryskill in the art.

It can be appreciated that, in certain aspects of the invention, asingle component may be replaced by multiple components, and multiplecomponents may be replaced by a single component, to provide an elementor structure or to perform a given function or functions. Except wheresuch substitution would not be operative to practice certain embodimentsof the invention, such substitution is considered within the scope ofthe invention.

The examples presented herein are intended to illustrate potential andspecific implementations of the invention. It can be appreciated thatthe examples are intended primarily for purposes of illustration of theinvention for those skilled in the art. There may be variations to thesediagrams or the operations described herein without departing from thespirit of the invention. For instance, in certain cases, method steps oroperations may be performed or executed in differing order, oroperations may be added, deleted or modified.

Furthermore, whereas particular embodiments of the invention have beendescribed herein for the purpose of illustrating the invention and notfor the purpose of limiting the same, it will be appreciated by those ofordinary skill in the art that numerous variations of the details,materials and arrangement of elements, steps, structures, and/or partsmay be made within the principle and scope of the invention withoutdeparting from the invention as described in the claims.

What is claimed is: 1-14. (canceled)
 15. An optical coherence tomographysystem comprising a drive waveform source comprising a memory, anoutput, and one or more function generators configured to (a) generate afirst harmonic wave, (b) generate a second harmonic wave (c) generate athird harmonic wave; and (d) combine the first, second, and thirdharmonic waves such that a drive waveform is generated and stored in thememory; and a light source comprising a tunable filter having anasymmetric sweep response, the tunable filter in electricalcommunication with the output and configured to receive the drivewaveform from the output, wherein the second harmonic wave has a firstphase value configured to cause a peak RF frequency for a backward scanof the tunable filter and a peak RF frequency for a forward scan of thetunable filter to differ by less than about 15%.
 16. The system of claim15 wherein the drive waveform source is a control system, a processor ora circuit.
 17. The system of claim 15 wherein the light source is alaser.
 18. The system of claim 17 wherein the tunable filter is a MEMStunable filter.
 19. The system of claim 17 the tunable filter is apiezoelectric tunable filter.
 20. The system of claim 17 wherein thethird harmonic wave has a second phase value configured to reduce orminimize a peak RF OCT signal frequency for a backward scan and aforward scan of the tunable filter.
 21. The system of claim 17 whereinthe third harmonic wave has a first amplitude value configured to reduceor minimize a peak RF OCT signal frequency for a backward scan and aforward scan of the tunable filter.
 22. The system of claim 15 whereinthe first phase value is generated by searching for an extremum in a twodimensional space that includes forward and backward scan data measuredwith respect to the light source as the tunable filter is swept.
 23. Thesystem of claim 15 wherein the first amplitude value is generated bysearching for an extremum in a two dimensional space that includesforward and backward scan data measured with respect to the light sourceas the tunable filter is swept.
 24. The system of claim 17 wherein aneffective duty cycle of the laser is greater than about 90% of a sweepperiod of the light source when collecting OCT data.
 25. The system ofclaim 17 wherein a forward scan duration of the tunable filter is aboutequal to a backward scan duration of the tunable filter.
 26. The systemof claim 15 wherein a ratio of a forward scan duration to a backwardscan duration ranges from about 0.8 to about 1.2 when the drive waveformis applied to the tunable filter.
 27. The system of claim 15 wherein aratio of a peak RF fringe frequency during a forward scan to a peak RFfringe frequency during a backward scan duration ranges from about 0.9to about 1.1 when the drive waveform is applied to the tunable filter.28. The system of claim 15 wherein a sweep response in a forward scandirection of the light source and a sweep response in a backward scandirection of the light source is substantially symmetric when thetunable filter receives the drive waveform.
 29. The system of claim 23wherein the two dimensional space is an amplitude and phase space. 30.An optical coherence tomography system comprising: a swept light sourcecomprising a tunable filter comprising an input, the tunable filter inoptical communication with a sample arm of an interferometer, whereinthe tunable filter is drivable bidirectionally and has an asymmetricsweep response; a control system comprising a non-transitory memory andan output, wherein the output is in electrical communication with theinput, the control system configured to drive the tunable filter in oneor more directions; and a drive waveform stored in the non-transitorymemory, the drive waveform configured to substantially symmetrize andsubstantially linearize a sweep response of the swept light source,wherein an effective duty cycle of the swept light source is greaterthan about 90% of a sweep period of the swept light source whencollecting optical coherence tomography data.
 31. The system of claim 30wherein a ratio of a forward scan duration to a backward scan durationranges from about 0.8 to about 1.2 when the drive waveform is applied tothe tunable filter.
 32. The system of claim 30 wherein a ratio of a peakRF fringe frequency during a forward scan to a peak RF fringe frequencyduring a backward scan duration ranges from about 0.9 to about 1.1 whenthe drive waveform is applied to the tunable filter.
 33. The system ofclaim 30 wherein a forward scan duration of the tunable filter is aboutequal to a backward scan duration of the tunable filter.
 34. The systemof claim 30 wherein a sweep response in a forward scan direction of theswept light source and a sweep response in a backward scan direction ofthe swept light source is substantially symmetric when the tunablefilter receives the drive waveform.
 35. The system of claim 30 whereinthe stored drive waveform is generated by superimposing two or moreharmonic waves, the two or more harmonic waves defined using a phasevalue and an amplitude value.
 36. The system of claim 35 wherein thephase value is selected to reduce or minimize a peak RF OCT signalfrequency for a backward scan and a forward scan of the tunable filter.37. The system of claim 35 wherein the amplitude value is selected toreduce or minimize a peak RF OCT signal frequency for a backward scanand a forward scan of the tunable filter.